Together with the recent advancements in scientific technologies, micro structured members are being actively developed in the fields of microactuators, electronic devices, optical devices and the like, and are being in commercial use in various microsensors, microprobes, thin film magnetic heads and ink jet recording heads.
For producing such micro structured member, various methods are utilized such as a stamper method, a dry etching and a photolithography. Among these, the pattern formation by a photolithography utilizing a photosensitive resinous material has an advantage that a desired pattern can be obtained simply with a high precision.
As the photosensitive resin composition to be employed for such purpose, a negative type cationic photopolymerizable epoxy resin composition may be employed in consideration of a pattern forming property, a chemical resistance and a heat resistance.
The preparation of a micro structured member utilizing a cationic photopolymerizable epoxy resin is conducted in the following manner. After it is uniformly coated for example by a coater on a substrate, it is subjected to an exposure process and a PEB process whereby a crosslinking reaction by epoxy groups proceeds only in an exposed area, and, in a development process, a surface irregularity (pattern) is formed by a difference in a dissolving rate in a developing solution, between an exposed area and an unexposed area.
For such cationic photopolymerizable epoxy resin composition, various characteristics are required when it is used in the aforementioned application. Such characteristics specifically include followings:
coating characteristics: uniformity in plane at coating;
high solid content formation: low viscosity and high solubility (ability for forming a thick film of tens of micrometers);
film characteristics: chemical resistance, excellent mechanical strength (high toughness) and adhesion to underlying surface;
photosensitive characteristics: high resolution, high dimensional stability, high sensitivity, and wide process margin;
productivity: high industrial productivity capable of enabling a mass production.
Particularly in the field of a liquid discharge head applied as an ink jet recording head and the like, the cationic photopolymerizable epoxy resin is normally used as members for forming a discharge port for a liquid such as ink and a liquid flow path, and is used in a state adjoined to a substrate such as of silicon. Such member is constantly in contact with an ink (which is generally constituted principally of water and is often not neutral), and, as a film, there are required a liquid resistance such as an excellent ink resistance, a mechanical strength and a high adhesion. Also it is recently made clear that the shape of the discharge port influences precision of liquid discharge, and high photosensitive characteristics are required in order to form a desired pattern with a high precision.
Various cationic photopolymerizable epoxy resin compositions have hitherto been proposed in order to meet these requirements, but each of these is not satisfactorily in meeting all these requirements, even though being excellent in some of the characteristics.
An ordinary epoxy resin is obtained by a polymerization reaction of an epoxy compound, including several epoxy groups, as a monomer. Therefore, the polymerization proceeds with simultaneous reactions of the plural epoxy groups, thus constructing a disorderly three-dimensional structure. In order to improve the coated film characteristics of the cationic photopolymerizable epoxy resin, as it is dependent on a primary molecular weight of a base resin in the composition, it is desirable to increase the molecular weight of the epoxy resin thereby improving the characteristics of the coated film. However, an increase in the molecular weight often results in a significant increase in the viscosity of the solution and in an evident decrease of the solubility in the solvent, whereby the improvement in the characteristics is inevitably restricted.
Also for such target, the improvement in the coated film characteristics may be intended by a method of controlling the molecular weight of the epoxy resin, executing a solution coating in a state where the molecular weight is maintained at about oligomers that can maintain a solubility in the solvent, and causing a crosslinking reaction of plural epoxy groups at a curing operation. It is often difficult, however, to form a thick film of several ten to several hundred micrometers, because of the molecular weight and the viscosity.
On the other hand, highly branched polymers are specific polymers having many branching points in a main chain, and are recently attracting attention as polymers involving unknown possibilities. As a feature thereof, it is made clear that they show a low viscosity in a solution and have a high solubility in various solvents, owing to a fact that an intermolecular entanglement is reduced in comparison with ordinary polymer.
The highly branched polymers are classified, according to the structure thereof, into a dendrimer represented by a following formula (2) and a hyper branched polymer represented by a following formula (3):

The dendrimer is, as represented by the formula (2), a polymer having a star-like structure. However, the synthesis of dentrimer requires a process of, starting from a core portion, repeating a protection, a bonding and a de-protection of a monomer, and is therefore cumbersome. It therefore involves a problem in the productivity and is unsuitable as a material for practical use.
The hyper branched polymer is investigated in various manners because of the attractive characteristics as described above. Various hyper branched polymers and producing method therefor are reported for example in Japanese Patent Application Laid-Open No. 2000-256459 and Japanese Patent Application Laid-Open No. 2001-114825.
Also Japanese Patent Application Laid-Open N 2004-331768 discloses an example of employing a hyper branched epoxy resin in a cationic photopolymerizable epoxy resin composition. More specifically, a hyper branched epoxy resin is synthesized, as represented by a following formula (38), by a bimolecular reaction of (A2+B3) type of a phenol compound and an epoxy compound:

However, the hyper branched polymer obtained by the bimolecular reaction of (A2+B3) type may generally be insufficient in a level of branching per unit polymerization degree and in a number of terminal epoxy groups. Therefore, the properties resulting from these factors may be insufficient for use in the cationic photopolymerizable epoxy resin.
Also the hyper branched polymer obtained by the bimolecular reaction of (A2+B3) type has a property, in comparison with an AB2 type, that the molecular chain grows in multiple directions at the growth of the molecular chain, and does not provide a fan-shaped final form, thus causing a gelation at the polymerization.
As an example, let us consider monomers of A2 (following formula (6))+B3 (following formula (7)) type and an intermediate (following formula (8)) prepared by an addition reaction of one molecule each. A group A in the compound of formula (6) and a group A′ in the compound of formula (8) are same, but are represented separately for the purpose of clarity:

As to the reactivity of each functional group in the compounds (6), (7) and (8), at a stage where the compound (8) has grown to a certain level, it is desirable that the compound (8) grows as a polymer by a reaction of B′ with A′. However, in the case that A′ of the compound (8) reacts the polymerization reaction does not proceed in a fan shape but the molecular chain grows in multiple directions, thereby possibly causing a gelation at the polymerization. Also a difficulty exists in causing the epoxy groups to remain efficiently in terminal ends of the resulting polymer. For this reason, in case of synthesizing a hyper-branched type polymer of the A2+B3 type, it is necessary to strictly design the reactive groups of the two monomers in terms of reactivity, and a restriction is involved in the selection of the monomers.